The present invention relates to fire detection apparatus for an extensive protection area, with a plurality of fire detectors connected in pairs to a fire detection centre by means of at least two line systems, whereby the fire detection centre is set up in such a way that it signals a fire alarm on simultaneously receiving via both line systems a signal released by at least one fire detector in each system.
The fire detectors can be of many types which react to particular fire phenomena or to a predetermined temperature maximum or differential. The associated fire detection centre can have two known switching or signal equipments interconnected in an AND circuit and which react to the alarm current of a fire detector.
Various types of fire detectors are known for reacting to certain fire phenomena. These include detectors which react to a predetermined maximum temperature and detectors which react to a temperature differential. The fire detection centre associated with such detectors can have two known switching or signal units interconnected in an AND circuit and which react to the alarm current of a fire detector.
However, pneumatic fire detection systems have the disadvantage that they are triggered even if small leaks occur, such as from slow corrosion. As the system cannot be monitored, it is impossible to automatically detect such defects. In addition, the discovery and location of the faulty point is extremely difficult and correspondingly costly. When such pneumatic fire detection systems are used in a vehicle tunnel, for example, this problem is exacerbated by stresses resulting from pressure waves and vibrations caused by the vehicles passing through and by local and time-based temperature fluctuations, together with rock movements which can lead to the fracture of pipe lines, rendering the system inoperative or causing false alarms.
It is known that these disadvantages can be partly obviated by electrical fire detection systems in which several fire detectors are distributed individually over the protection area and are connected in groups to a signal station by means of common lines. Such fire detectors can be of any desired type, e.g. ionization smoke detectors, stray detectors, combustion gas sensors, flame detectors or temperature detectors operating with bimetallic strips, aneroid diaphragms, or temperature-sensitive semiconductors which emit an alarm signal either when the corresponding fire phenomenon has reached a certain intensity or when it reveals a rapid time-based change. However, it is a disadvantage of the various types of fire detectors that they have a tendency to give a false alarm signal without there being any cause for an alarm. This can either be caused by the actual characteristics of the fire detector as a result of ageing, dust contamination, etc., or can result from external influences which may only simulate a fire.
However, particularly in vehicle tunnels false fire alarms cannot be tolerated, due to the measures which are initiated whenever there is an alarm, e.g. total traffic stoppage, summoning of police and fire brigade, etc. Therefore, attempts have been made to prevent the tendency towards false alarms of such electrical fire detection systems through the use of two parallel line systems. FIG. 1 shows such a fire detection arrangement in which a plurality of fire detectors A1, A2 . . . A6 are arranged at regular intervals along a tunnel T. The individual detectors are alternately connected by means of a line L1 or a second line L2 to a fire detection centre C which releases an alarm signal on simultaneously receiving an alarm signal from a fire detector connected to each of the two lines L1 and L2. Use is thereby made of the fact that for example in the case of a fire between fire detectors A1 and A2, both of the detectors are activated. The giving of false alarms can be largely eliminated by this known scheme, which is called two-loop dependence. However, a disadvantage of this method is the varying sensitivity of the system as a function of the fire location point. If, for example, the centre of the fire is located directly below a detector A2, little influence is exerted on adjacent fire detectors A1 and A3, whereas detector A2 is influenced to a very great extent. However, an alarm signal can only be released if a sufficiently great influence is exerted on adjacent fire detectors A1 or A3. However, in the case where a fire centre is located between two fire detectors, e.g. between A1 and A2, an alarm signal is released much earlier, because then both detectors A1 and A2 are sufficiently influenced by the fire much earlier.
In order to increase the security against false alarms, it has already been proposed to arrange two sensors at each measurement point which react to a different fire phenomenon, e.g. simultaneously to smoke and temperature rise or to smoke and combustion gases. In the example shown in FIG. 2, at a first location there are provided in each case a detector of the first type A1 and a detector of the second type B1, with corresponding detectors A2 and B2, etc., at following monitoring locations, whereby the detectors of the first type A1, A2, etc., are connected via a line system L1 to the fire detection centre C, and the detectors of the second type B1, B2, etc., are connected thereto via a second line system L2. Centre C releases a signal only on receiving a signal via both lines from at least one of the fire detectors, i.e. if several fire phenomena occur simultaneously. This permits a relatively reliable differentiation between a genuine fire and other disturbance variables, but does not solve the problem of differing sensitivity depending on the location of the centre of the fire relative to the individual pairs of fire detectors. In such systems, it was therefore necessary to use a maximum number of fire detectors in order to obtain an adequate sensitivity with minimum fluctuations over the entire length of the tunnel.
It would be desireable, then, to provide an electrical fire detection system having a high operational reliability and low tendency to false alarm, while using a reduced number of fire detectors. At the same time, the response sensitivity should have optimum constancy over the entire protection area, independently of the distance of the centre of a fire from the fire detector.